Various packaging techniques are known for microelectronic circuits. In one type of microelectronic circuit, a printed circuit board containing interconnected circuit components which have been inserted and soldered thereon has an array of terminal pins which are attached to the printed circuit board usually along one or more edges thereof. The circuit board is contained within a plastic shell or case, and is supported within the shell on standoffs to retain the board in spaced relation to the inner top surface of the shell to provide clearance space for the components which are mounted on the board. An epoxy or other encapsulate is thereafter provided to fill the shell interior and enclose the circuit board therein. A bottom plate may be included over the encapsulate and through which the terminal pins extend. The resulting package is a small rectangular box with terminal pins outwardly extending from the bottom thereof.
In the type of microelectronic circuit known as a hybrid circuit, a conductive and/or resistive pattern is provided by thick or thin film techniques on one or both surfaces of a ceramic substrate, and monolithic integrated circuit chips and other component chips are affixed to mounting areas on the substrate and interconnected to selected portions of the deposited pattern by wire bonding. For hermetic packaging, the hybrid circuit is usually mounted in a metal housing which is sealed by brazing or welding to provide a moisture and gas tight enclosure.
According to a widely employed hybrid circuit packaging technique, a ceramic substrate containing the hybrid circuit on one surface thereof is mounted within the opposite plain surface on a metal header which has an array of terminal pins affixed thereto usually in two rows along respective opposite sides of the header. The terminal pins extend through the header by means of electrically insulating glass-to-metal seals, and have portions extending outwardly from the surface of the header on which the ceramic substrate is mounted. The hybrid circuit is connected by wire bonding between selected portions of the circuit and corresponding ones of the connection portions of the terminal pins. A lid usually of metal is disposed over the mounted circuit and is brazed or otherwise bonded to the periphery of the header to complete the package.
The hybrid circuit package described above provides suitable hermetic sealing and mechanical protection for the circuit but is of a cost which can be a significant portion of the overall manufacturing cost of the product. To produce a less costly package, hybrid circuits have been fabricated with conformal coatings of encapsulating material; however, such coatings are often not reliable since they are not impervious to moisture leakage which can enter the package and degrade or wholly destroy circuit performance.